Table of contents

The calculation and measurement of the properties of thermal plasmas in mixtures of different gases are reviewed. The calculation of composition, thermodynamic properties and transport coefficients is described. Particular attention is given to the calculation of diffusion coefficients, which is a significant problem in mixed-gas plasmas. The combined diffusion coefficient formulation is shown to be a useful method for the treatment of diffusion. Computational fluid dynamic modelling of thermal plasmas in gas mixtures is considered, using the examples of demixing in welding arcs, the turbulent mixing of atmospheric air into a plasma jet and a plasma waste destruction process. Diagnostic techniques for mixed-gas plasmas, in particular emission spectroscopy, laser scattering and laser-induced fluorescence, are discussed.

Ion implantation conditions are considered during which the energy, dissipated in the collision cascades, is low enough to ensure that the defects, which are generated during these collisions, consist primarily of vacancies and interstitial atoms. It is proposed that ballistic self-annealing is possible when the point defect density becomes high enough, provided that none, or very few, of the interstitial atoms escape from the layer being implanted. Under these conditions, the fraction of ballistic atoms, generated within the collision cascades from substitutional sites, decreases with increasing ion dose. Furthermore, the fraction of ballistic atoms, which finally end up within vacancies, increases with increasing vacancy density. Provided the crystal structure does not collapse, a damage threshold should be approached where just as many atoms are knocked out of substitutional sites as the number of ballistic atoms that fall back into vacancies. Under these conditions, the average point defect density should approach saturation. This model is applied to recently published Raman data that have been measured on a 3 MeV He⁺-ion implanted diamond (Orwa et al 2000 Phys. Rev. B 62 5461). The conclusion is reached that this ballistic self-annealing model describes the latter data better than a model in which it is assumed that the saturation in radiation damage is caused by amorphization of the implanted layer.

Freestanding polyimide (PMDA-ODA) thin films (thickness = 1 to 30 µm) and spherical shells (diameter = 950 to 1100 µm, wall thickness = 0.7 to 11.0 µm) have been fabricated by vapour deposition polymerization (VDP). The deposition process was optimized to obtain smooth and uniform deposition composed of equal-molar PMDA and ODA. The tensile properties, including elastic modulus, tensile strength, and elongation at break, were determined by buckling and bursting the shells and using a nanoindentation test on the films. The gas permeability was measured for He, H₂, D₂, O₂ and N₂. The VDP polyimide possessed distinct properties from solution-cast PMDA-ODA polyimide (Kapton^®), including better tensile properties and lower permeability, and was insoluble in concentrated sulfuric acid. The differences in the properties were attributed to the presence of physical or chemical cross-linking in the VDP polyimide.

The effects of thermal imidization conditions on the properties were studied, including different heating rates (0.1 to 1.0 °C min^-1), atmospheres (air and N₂), and imidization durations (1 to 6 h). Varying the imidization conditions effectively modified the tensile strength, elongation at break and permeability but had little effect on the elastic modulus. Infrared spectroscopy and solubility tests indicated that the changes in the properties resulted from changes in the molecular weight or degree of cross-linking.

Magnetization reversal has been studied in a 5 nm thick Ni₈₀Fe₂₀ film under the action of a pulsed magnetic field, for pulse durations between 2-102 ns. Fast pulses were applied to the sample from a thin-film microstrip transmission line and magnetization changes were measured using a continuous wave laser magneto-optic Kerr effect magnetometer. The switching field was found to increase as the pulse duration decreased, in qualitative agreement with simple models. More unusually, the switching field was also found to increase as a function of a quasi-static bias field in excess of the value of that bias. This effect is stronger for the shorter pulses. Evidence is presented which suggests that this is due to rapid relaxation of the magnetization after the termination of the pulsed field.

The influence of a partial boron substitution on the crystal structure, Curie temperature, easy magnetization direction and magnetostriction of Tb_0.7Pr_0.3Fe₂ has been studied. The matrix of Tb_0.7Pr_0.3Fe₂ consists of the (Tb, Pr)Fe₂ phase with a cubic MgCu₂-type structure coexisting with a large amount of the (Tb, Pr)Fe₃ phase with a rhombohedral PuNi₃-type structure. The introduction of boron can effectively restrain the emergence of the iron-rich phase and thus the Tb_0.7Pr_0.3(Fe_1-xB_x)₂ alloys are essentially single phase compared with the boron free material. The linear magnetostriction at room temperature λ_a = λ_∥-λ_⊥ and T_C pronouncedly increase with the boron substitution.

In this work the effects of electron-ion collisions in the plasma column of a vacuum arc centrifuge are modelled using a perturbation technique. It is found that the model agrees reasonably with an earlier fluid simulation, in which ion viscosity effects were also included. Using the perturbed solutions, the axial evolution of the steady-state separation profile is resolved, showing that the effect of electron-ion collisions is to improve separative performance with increasing axial position. The conditions under which separative performance are optimized are suggested. The non-uniformity in the axial magnetic field of a vacuum arc centrifuge caused by plasma rotation is also investigated. It is found that the non-uniformity is weak for standard operating conditions.

Arc jets that spout from electrodes as a result of high-current fault arcs cause damage to the surrounding apparatus. This paper describes the results of the spectroscopic observation of arc jets of 10-50 kA rms ac arcs at the current peak. The electrodes were copper of 40 mm in diameter, and the gap length between the electrodes was 3.4 m. The temperature and the copper vapour concentration of the arc jet were determined by considering the self absorption of two spectral lines. As the arc current increased from 10 kA to 50 kA, the temperature did not change greatly (10 000-11 000 K), and the maximum copper vapour concentration (molar rate) in the arc jet increased from 0.03% to 3%.

A theoretical study of the ion density profiles resulting from the interaction between metallic plasma ejected from cathode spots and background neutral gas is presented. The model includes four different charge-state species for the metallic particles and also the gaseous ions, and a set of coupled continuity and momentum equations is solved. The electric field profile in the neutral plasma region (that is, outside the spot structure) is also obtained. The main included atomic processes are elastic collisions between heavy particles and neutral molecules, charge exchange between metallic ions and neutral gas and ionization and recombination (dissociative) of the gas by electrons. A simplified equation for the temperature of the metallic particles is also solved in order to evaluate the pressure gradient force on the metallic neutrals generated by charge-exchange processes.

Solutions for the profiles of the physical quantities involved in the problem are presented for a non-filtered copper arc with a discharge current of 100 A running in an oxygen ambient with background pressure values in the range 10^-4-5 Pa. The relative weight of the different atomic processes is also studied as a function of the gas pressure.

Low-density polyethylene (LDPE) filled with a small amount of titanium dioxide proves to be very suitable for studying the molecular motion of polyethylene by means of dielectric spectroscopy. The titanium dioxide acts as an oxidizing agent that introduces a small concentration of polar carbonyl groups, which does not alter molecular motion, but couples it to the alternating electric field.

The frequency and temperature dependences of molecular-motion processes in LDPE are determined from both isochronal and isothermal loss plots. In particular, the temperature dependence of the β process connected with the glass-rubber transition in polyethylene obeys the Vogel-Fulcher-Tammann-Hesse law, which indicates co-operative behaviour. For pure LDPE, a glass-transition temperature of -31 °C is determined dielectrically. It increases with increasing titanium dioxide content.

Explicit expressions for the effective pyroelectric, dielectric, piezoelectric, elastic and thermoelastic constants of a matrix composite have been obtained in closed form. A numerical analysis has been carried out of the concentration dependences of constants of a composite in which the BaTiO₃ ceramics is the matrix while the inclusions are represented by spheroidal ceramic particles of PZT-19. A case is examined when the matrix and the inclusions possess opposite polarization. At a certain concentration of PZT-19 the composite exhibits well pronounced pyroelectric properties without having any appreciable piezoeffect. At another concentration, it manifests marked piezoelectric properties, but does not possess the pyroeffect. Practical implementations of the results are discussed.

The sound velocities of vanadium at shock pressure ranging from 154 to 250 GPa were determined using transparent-window optical analyser techniques. A discontinuity in sound velocities at about 225 GPa may mark the partial melting under shock compression. The comparison between the measured sound velocity data sets above ~225 GPa and calculated values yields γ₀≈2.0 and the empirical expression γρ = γ₀ρ₀ is basically tenable. Additionally, shock temperatures along the principal Hugoniot of vanadium were also determined from interfacial radiation intensities according to Grover's ideal interface model. Thus the temperature at this solid-liquid phase transition was constrained to be round about 7800(±800) K on the basis of the measured Hugoniot temperatures, melting temperatures, and high-pressure sound velocity variations with pressure.

Laser induced pressure pulse space charge measurements were made on 1.5 mm thick plaques of high purity low density polyethylene equipped with vacuum-evaporated aluminium electrodes. Temperature differences up to 20 °C were maintained across the samples, which were subjected to dc fields up to 1.5×10⁷ V m^-1. Current density was also measured as a function of temperature and field. Space charge due exclusively to the temperature gradient was detected, with density of order 0.01 C m^-3. The activation energy associated with the transport of electrons through the bulk was calculated as 0.09 eV. However, measurements of the temperature dependence of the current in isothermal samples yielded activation energies in the range 0.9-1.3 eV. It is deduced that most of this larger activation energy is associated with charge injection at the electrodes. Electron mobilities not less than 2×10^-12 m² V^-1 s^-1 were inferred by combining the space charge and current density measurements.

A GaAs hemisphere has been used for the first time both as a probe and a solid immersion lens in a reflecting external electro-optic measuring system to substantially improve the spatial resolution and the voltage sensitivity of the system. The electric signals propagating on microstrip lines were detected successfully. In contrast with a conventional cubic probe under the same measuring conditions we found that the voltage sensitivity of the measuring system with the hemispherical probe was improved by a factor of nearly 1.7 to 3 mV Hz^-1/2, and the spatial resolution was improved by a factor of 4.5 to 0.6 µm. The improvements in the spatial resolution and voltage sensitivity are due to the increased effective number aperture and the total internal reflection in hemispherical GaAs probe, respectively.

A white organic light emitting diode (OLED) has been constructed by employing a new blue material and a red dye directly doped in the blue emitting layer. For comparison, another white cell with a blocking layer has also been made. The configurations of the devices are ITO/CuPc/NPB/JBEM(P):DCJT/Alq/MgAg (device 1) and ITO/CuPc/NPB/TPBi:DCJT/Alq/MgAg (device 2) where copper phthalocyanine (CuPc) is the buffer layer, N,N'-bis-(1-naphthyl)-N,N'-diphenyl-1.1'bipheny1-4-4'-diamine (NPB) is the hole transporting layer, 9,10-bis(3'5'-diaryl)phenyl anthracene doped with perylene (JBEM(P)) is the new blue emitting material, N,arylbenzimidazoles (TPBi) is the hole blocking layer, tris(8-quinolinolato)aluminium complex (Alq) is the electron transporting layer, and DCJT is a red dye. A stable and current independent white OLED has been obtained in device 1, which has a maximum luminance of 14 850 cd m^-2, an efficiency of 2.88 Lm W^-1, Commission Internationale de l'Eclairage coordinates of x = 0.32, y = 0.38 between 4-200 mA cm^-2, and a half lifetime of 2860 h at the starting luminance of 100 cd m^-2. Device 1 has a stability more than 50 times better than that of device 2.

A study of the effect of light deflection during diffusion studies of ethanol into agarose gel using holographic laser interferometry is presented. Furthermore it also demonstrates how a diffusive flux could give rise to a convective flux in holographic laser interferometry experiments. The convective and diffusive mass transfer is also theoretically compared in both a liquid phase and a gel phase for the ethanol-agarose system used.

The current study shows that errors due to light deflection in holographic laser interferometry are extremely small and can be neglected. It also shows the importance of designing the diffusion experiments to avoid natural convection. In gels the convective flow is cancelled by the friction forces between the liquid and the polymer network. However, in the liquid phase the natural convection could occur even though the density differences in the phase are small.